I've used 800-score test as a prep tool and they are ok, not great, but ok especially if you need extra practice. One advantage of these tests, especially when you are just starting to prepare, is that you can stop the clock until you actually think through the question and get it right, as time running out was one of the most frustrating things with PP or Kaplan
when I would run out of time and start to freak out.
After doing one or two with 'stopping a clock' I was more comfortable to move on and do the tests without the 'think-breaks'. I found that getting timing right was one of the most difficult things and had to work on it a lot.
As to the level of questions - you can have a look at their free test, the level is more or less the same for all of them. It's hard to compare them to the 'real' GMAT questions that you will encounter, as these will depend on your percentile.
What stage of preparation are you in?